This invention relates to an ink for use on cardboard picture mats and other unglazed paper surfaces; and more particularly to a pigment-containing ink especially useful in providing uniform, permanent lines on such paper surfaces whether colored or white.
Many inks are known which are quite satisfactory for drafting purposes or for drawing lines on any type of glazed or hard-finished paper. In such use, absorption and bleeding or spreading of the ink can be well controlled. However, in drawing lines on unglazed paper or paper having a more open, fibrous surface, problems of spreading or of adherence and permanence arise. The stock not only absorbs the ink inwardly from the surface but along the surface or perpendicular thereto as well.
The present invention provides an ink which is excellent for use on such unglazed paper surfaces. This ink is especially suitable for use in a ruling pen and when so used forms a line of constant width without buildup at start or finish. The ink comprises pigments which are permanent, so that it does not fade after application which is important in colored lines in picture-framing, for instance. This ink has the further advantage that it does not bleed or spread out, even when applied to such a porous surface as 100% rag paper. It is also an advantage in practice that the ink comprises a water vehicle so that the pen or brush which has been used to apply the ink can be cleaned with water. Thus, there are numerous advantages in that the solvent portion of the vehicle of the present composition is water, water being economical, readily available, nontoxic, not harmful to the handler and nonflammable.
The ink of this invention is excellently adapted to use in a ruling pen, but it can also be applied in other desired ways, for instance by brushing. The ink is opaque as applied, is scuff-resistant, and provides a uniform ruled line. In using the ink, if the pigment settles the ink is mixed or agitated to make a uniform suspension, but due to the particle size and the viscosity of the ink, the pigment, except for the gold ink, stays in suspension for a considerable time after agitation. The gold ink, however, because of the high specific gravity of the bronze pigment and the larger particle size, would require frequent agitation. The surface tension is of a value to hold a charge of ink in a ruling pen and to prevent the ink from spreading out when applied to a porous surface such as blotter or high quality porous papers. On the other hand, if the surface tension were too high, the ink would not flow out of the pen at all, and if too low the ink tends to drip out of a ruling pen, for instance. The desirable surface tension, therefore, is advantageously from 50 to 60 dynes per cm for satisfactory operation. The opacity of the dried ink is advantageous over the inks which use water- or spirit-soluble dyes because, in part, the opacity produces a more solid looking line. This provides more definition in drawn lines, for example, in picture framing. A primary advantage of pigment over dye is that the pigments we use are permanent. We have found that even the best commercially available dye-based inks fade considerably when lines drawn with them are exposed to sunlight through a window for as little as three days, whereas the pigments used in this invention do not fade.